Co-fused alumina-zirconia abrasives went into commercial use in the early 1960's and are described in U.S. Pat. No. 3,181,939.
Improvements in such abrasives are described in Rowse and Watson U.S. Application Ser. No. 386,718 now Pat. No. 3,891,408 filed Aug. 8, 1973, Page 3, line 10, which discloses the addition of up to 4% magnesia in a cofusion of alumina and zirconia containing from 35 to 50% zirconia. Coes U.S. Pat. No. 3,498,769 discloses co-fused spinel-zirconia abrasive materials (spinel being magnesium aluminate). German Published application No. 2,212,311, published Oct. 26, 1972, suggests the additions of magnesia, among other things, in unspecified amounts to alumina-zirconia fusions. Japanese Pat. Publication, No. 14959, of July 15, 1970 discloses adding lime or magnesia stabilized zirconia to bauxite, fusing the mixture in the presence of carbon, and solidifying and crushing the product. The crushed material is then calcined at 1300.degree. C to strengthen it. The zirconia is stated to be stabilized in the cubic crystallographic form, in order for it to tolerate the calcination operation.
Alumina-zirconia available commercially prior to this invention has included material which has contained minor amounts of magnesia (less than 0.5% MgO by weight) carried over as an impurity from the raw materials employed.